The nature and function of thought experiments, which have become an important part of philosophical and scientific discourse, are among the topics that have been researched with great interest not only by philosophers but also by historians of science, cognitive scientists and psychologists in recent years. Thought experiments, which we can trace back to the Pre-Socratics, are frequently encountered in the Middle Ages, especially in fields such as theology and natural philosophy. Imaginary scenarios designed by important figures in the Islamic world such as Ibn Sina, Ibn al-Haytham and Ghazali provide important information about the way thought experiments were used in the period. Ibn Sina was the first philosopher to try to explain the psychological basis of thought experiments. The fundamental question he focuses on is: What is the source of the premises used in thought experiments? According to Ibn Sina, the source of thought experiments is wahm, an internal, sensory faculty that has no exact English equivalent but is usually translated as estimative faculty. My aim in this article is to focus on the faculty of illusion that Ibn Sina introduced while creating the psychological infrastructure of thought experiments and to reveal the role that this faculty plays in the formation of thought experiments.